Systems designed for hot insertion or removal of hardware must deal with a number of issues in order to prevent disruptions or errors from occurring due to freshly inserted or removed hardware. Fundamental electrical problems arise during hot insertion or removal, such as instantaneous load changes on the system power supply or instantaneous load changes on interconnected signals between circuit cards. Interconnected signals may also be disrupted by mechanical chattering when a circuit card's mating connector makes or breaks contact with the system. Signal disruption may also arise from the indeterministic behaviour of the electrical device driving or receiving the interconnected signal when its power supply is ramping-up during card insertion or when its power supply subsides during card removal. In addition, hot insertion or removal of cards may give rise to spurious behaviour on signals leading to their misinterpretation.
Such unpredictable signal behaviour is undesirable and can lead to system fault conditions, especially for low level signals. Low level signals are regular digital signals that are not encoded or protected in any way, and are used for basic functions such as card resets, circuit enables, and arbitration. Many solutions attempt to deal with each of the individual possible causes of signal disruption. For example, one or more recessed connector pins can be used to indicate that a card is fully inserted by detecting latent mating ground connections. The resultant signal acts as a qualifier for other signals or electrical circuits, essentially indicating that the card is fully inserted. This solution deals with mechanical connection problems. Another solution is to carefully select particular electrical circuits for driving or receiving interconnected signals.
While some of these solutions appear to be successful at addressing all the issues, problems often arise given the number of variables and the unpredictable behaviour of signals during transient states. Other solutions only address some of the issues, leaving other issues to chance.
A method which added integrity and robustness to the behaviour of low level signals in general, without being limited to particular signals or problems, would reduce spurious signals and allow more predictable behaviour of systems. Given the common nature of low level signals, such a method should also be simple and inexpensive.